The invention relates to a method for controlling a heating device for heating a component, in particular an exhaust gas sensor. The invention further relates to a correspondingly designed control device and a motor vehicle that comprises a control device of this type for controlling a heating device for heating a component, in particular an exhaust gas sensor.
It is known to control or to regulate internal combustion engines in dependence upon a composition of their exhaust gases, wherein the corresponding exhaust gas component is measured by means of a suitable exhaust gas sensor. In particular, the air-fuel ratio with which the engine is being driven is regulated in that the oxygen content of the exhaust gas is measured by means of a lambda sensor in the exhaust gas tract. The lambda sensor provides a sensor signal that is dependent upon the oxygen content of the exhaust gas, which sensor signal is usually a sensor voltage. However, the operational readiness of the exhaust gas sensor requires a specific minimum temperature of for example 300 to 400° C.
Currently available lambda sensors comprise efficient heating elements in order for the operational readiness of the sensor to be achieved rapidly upon activation of said lambda sensor and thus to ensure an emission-optimized engine control. This has the side effect that it is necessary to control precisely the heating voltage and the period of time that heat is applied. Otherwise, the sensor can be damaged as a result of high temperatures or thermal stresses. For this reason, maximum permissible heating voltages are generally defined and said voltages must not be exceeded during the heating strategy. Moreover, it is known to specify a maximum permissible period of time during which the sensor may be subjected to the maximum heating voltage. Moreover, the permissible period of time that heating can be provided at a maximum voltage can be made dependent upon additional criteria, for instance on the external temperature or on the preceding period of time during which the sensor is not heated, which renders it possible to take into consideration the sensor temperature at the commencement of the heating process. This is necessary owing to the fact that damage can also occur as a result of heating voltages that are lower than the maximum permissible voltage, if they are applied over longer periods of time.
Currently available lambda sensors may be heated for example using a maximum voltage of 14V in the case of a likewise specified maximum period of time. However, even if a sensor of this type is to be heated permanently using 13V, a maximum period of time is to be maintained and if said time limit is exceeded, it is to be expected that the sensor will be damaged. Although this maximum period of time is somewhat greater than the maximum period of time in the case of a 14V heating voltage, it must still be taken into consideration.
It is known from DE 10 2005 006 760 A1 to conduct the heating of lambda sensors in different phases, wherein in the initial phase, the heating voltage is rapidly brought to a high voltage, preferably to the full operational voltage, and is subsequently reduced in steps or in a ramp-like manner. In this manner, the sensor is to be heated as rapidly as possible but without being damaged. In addition, it is possible even prior to starting the engine, for instance when opening the driver's door, to provide preliminary heating using an extremely low heating voltage, for example 2V.
DE 10 2005 020 363 A1 also proposes that the lambda sensor is pre-heated even prior to starting the engine, for example upon activating the door contact. Initially, the sensor is heated using a higher initial voltage with a high rate of temperature increase to a temperature above the dew point temperature but below a thermal shock temperature so that existing condensation water can evaporate. If the first temperature is achieved, heating is continued using a reduced heating voltage in order to bring the sensor up to the target temperature at a lower rate of temperature increase yet still providing protection against over-heating. In order further to counteract any excessive heating, the duration of the heating process is limited.
DE 102 29 026 A1 describes an electrical circuit arrangement for heating a lambda sensor by means of a power output stage in the form of a field effect transistor (FET). The voltage drop is measured by way of the heating resistor and the FET. The power output stage i.e. the gate electrode of the FET, is controlled in dependence upon the feedback signal that is produced in this manner so that to a great extent compensation is made for the voltage that is dropping at the heating consumer. As a result, a variable current limitation is achieved and any uncontrolled overheating that could otherwise destroy the sensor heating arrangement is avoided.
Current strategies for heating sensors are confronted with the problem that it is not possible to predetermine in a precise manner the permissible heating parameters (maximum voltage and the maximum period of time of the maximum heating, but in particular the maximum period of time) in dependence upon different parameters. As a consequence, the sensor is either heated excessively or the permissible parameters are specified with such a large safety margin that the operational readiness of the sensor is achieved later than necessary.